1. Field of the Invention
The invention relates to a system and method of storing heat in a concentrating solar power plant. The system can use a compressed, preferably non-flammable gas, e.g., air or carbon dioxide (CO2), as the heat transfer medium.
Also, according to the invention, a large number of solar thermal collectors can be arranged in a row and the gas is passed through them at high velocity. As sunshine is not constant but the demand for electricity is ongoing, the solar thermal collectors are large enough to produce the desired total power output per day. During peak periods of sunshine, the heated gas from the solar thermal collectors is passed to the power plant as needed and the cool gas exiting the power plant is recycled through the collector back to the power plant. Any excess hot gas not needed in the power plant is passed through one or more heat storage tanks that are filled with a heat resistant solid filling.
The invention also relates to a heat storage system which can utilize one or more tanks. When solar insolation is insufficient, the hot gas can be supplied to the power plant by a secondary compressed gas cycle in which cool compressed gas is circulated in a reverse direction through the heat storage tanks to the power plant and back again through the heat storage tanks. This allows for efficient heat storage, which is equivalent to storage of electricity, and makes it possible for a solar power plant to be designed for any specific load schedule desired.
2. Discussion of Background Information
A critical advantage of concentrating solar power plants is that they can be provided with energy storage and can, therefore, operate with constant output, despite variations in sunshine during the day. The hot heat transfer fluid is then used to heat a conventional steam power plant and any excess hot heat transfer fluid not needed immediately can be stored as if it were a conventional fuel. However, the available heat transfer fluids for trough collectors are not only expensive, they also limit the thermal efficiency because they do not withstand temperatures above 800° F. Additionally, these fluids are typically also flammable. The solar power plants can be designed to perform like a conventional steam power plant in which the power output can be varied over a large range (see Shinnar, R., Citro, F.; “Solar Thermal Energy: The Forgotten Energy Source”, in publication in Technology in Society and Shinnar, R., Citro, F.; “A Road Map to U.S. Decarbonization”, Science, vol. 313, p. 1243, (Sep. 1, 2006)). The disclosures of these documents are hereby expressly incorporated by reference in their entireties. This is possible because heat is usually received by a hot heat transfer fluid flowing through the solar thermal collectors. (see Sargent & Lundy, “Assessment of Parabolic Trough and Power Tower Solar Technology Cost and Performance Forecasts”, SL-5641, (2002) and Shinnar, R., “The Rise and Fall of Luz”, ChemTech 23, p. 50-53 (1993). The disclosures of these documents are hereby expressly incorporated by reference in their entireties.
One solution proposed in Sargent & Lundy, “Assessment of Parabolic Trough and Power Tower Solar Technology Cost and Performance Forecasts”, SL-5641, (2002) is to use a salt with a low melting point. This accommodates higher temperatures (up to 1050° F.) but introduces another severe problem. Molten salts can solidify as they cool at night. As a result, heat tracing is required and operational problems can ensue. The present design of trough collector uses a Dowtherm-like fluid in the collector and molten salts in the storage tank. This limits the maximum temperature to less than 800° F., a severe limitation in thermal efficiency which the invention removes through storage at practically any temperature at which the power plant can operate.
The invention solves one or more of the problems associated with conventional solar power plants; it is simple in design, is more robust, is cheaper and lacks one or more of the disadvantages of conventional solar power plants with storage.